Electrowetting is electrostatic control of the contact angle between a liquid and a solid. A voltage difference applied between a conductive liquid and a conductive substrate reduces the interfacial energy, which increases the degree of wetting of the substrate by the liquid. Electrowetting can be applied to move and shape volumes of liquids. For example, when a water droplet is present on a hydrophobic surface, the contact area between the two is minimized. However, when a suitable voltage difference is applied between a first electrode that is present underneath the hydrophobic surface and a second electrode that is placed in the water droplet, the water droplet spreads over the hydrophobic surface (in other words, the hydrophobic properties of the surface appear to decrease). When the voltage difference is removed, the water droplet returns to its original state.
An electrowetting device is a device that, in operation, makes use of the electrowetting effect. Electrowetting devices are used in a wide range of applications, including variable-focus lenses (such as a variable-focus contact lens), electronic displays, switches for optical fibers, and microelectromechanical systems (such as microfluidic devices and lab-on-a-chip devices).
Electrowetting devices typically comprise a cell wherein an electrowetting composition comprising two immiscible fluids, one of which is polar and/or electrically conductive can be manipulated by applying a voltage difference between two electrodes.
To prevent electrolysis of the electrowetting composition, one of the electrodes may be separated from the electrowetting composition by a dielectric medium (in the remainder of this text simply called a dielectric). Such an electrowetting device is usually referred to as an electrowetting-on-dielectric (EWOD) device. The other electrode may be in direct contact with the polar and/or electrically conductive liquid, or it may be capacitively-coupled to this liquid.
EWOD devices typically have a dielectric that comprises an amorphous fluoropolymer (for example, Teflon® AF), silicon dioxide (SiO2), or parylene (a poly(p-xylylene) polymer that can be deposited by chemical vapor deposition), or a stack of these layers, having a thickness in the order of micrometers so that a relatively high voltage difference (on the order of 100 V) is required to operate these devices.
To reduce device size and/or power consumption and to be able to use standard electronic components, there is a need for EWOD devices that can be operated at lower voltages.
The required operating voltage of an EWOD device can be reduced by increasing the dielectric constant and/or by decreasing the thickness of the dielectric, thus increasing the capacitance of the dielectric.
Reducing the thickness of the dielectric leads to a lower operating voltage, but also to a larger electric field inside the dielectric, and to a higher probability for the occurrence of pinholes in the dielectric. Below a certain minimum layer thickness, electric breakdown of the dielectric (also referred to as dielectric breakdown) occurs before the desired electrowetting effect is obtained.
EWOD devices that can be operated at a reduced voltage are disclosed in US-2006/0221458 and US-2008/0100905, respectively. These known EWOD devices comprise a container with a conductive or polar liquid material, and a first electrode for applying a voltage to the conductive or polar liquid material through a dielectric. The dielectric is a metal oxide layer formed by anodizing the first electrode. The thickness of the dielectric can be adjusted easily and accurately by adjusting the voltage that is applied during the anodization process. Moreover, metal oxides of comparatively high dielectric constant can be formed by anodizing aluminum and tantalum. Furthermore, such metal oxides can be made into pinhole-free layers.
A drawback of the known EWOD devices is that over time dielectric breakdown may still occur, for instance as a result of mechanical stress, dielectric fatigue or ion injection from the fluids during the lifetime of the devices.